This invention relates to semiconductor devices, and more particularly to an improved electrode structure for semiconductor devices.
It is well known to use a polycrystalline silicon layer doped with impurities as a diffusion source and also a part of an electrode. FIGS. 1A to 1E show a well known semiconductor device and a method for producing the device.
In FIG. 1A, a silicon substrate 10 is doped with P type impurities, for example boron. On substrate 10, an insulation film 11 made of silicon oxide, for example, is formed generally by thermal oxidation of the surface region of substrate 10. A second insulation film 12, such as silicon nitride film, is formed on silicon oxide film 11, and then, is partly removed by photo etching so as to provide opening 14. The remaining portions of silicon nitride film 12 are used as the mask in the subsequent steps of providing silicon oxide film 11 with opening 15 to expose the surface of the substrate 10 (FIG. 1B). In this step, the silicon oxide film 11 is etched laterally as well as vertically, so silicon nitride film 12 overhangs silicon oxide film 11. Then a polycrystalline layer 16 doped with impurities producing N-type semiconductors, such as arsenic, is applied by chemical vapor deposition (CVD) to cover openings 14 and 15. The structure is then heated to diffuse the impurities of polycrystalline layer 16 into substrate 10 to form a diffused region 17 (FIG. 1D). Then an activating treatment, using fluoric acid or an aqueous solution of ammonium fluoride, is performed, and after that a metal electrode 18 is formed.
In this prior art device and method, the following problems exist. First, the overhanging portion of polycrystalline layer 16 tends to be broken, particularly when layer 16 is thin. Furthermore,in the activating treatment for improving the ohmic contact between metal electrode 18 and the polycrystalline layer, silicon oxide film 11 is further etched laterally, so a short-circuit between the metal electrode and the silicon substrate occurs more easily and often.
Second, to overcome the above-mentioned short-circuit problem, it is necessary to prepare or form a thick polycrystalline layer as shown in FIG. 2. However, this polycrystalline layer is used as a diffusion source, so this layer should be doped with impurities at high concentration. But the growth rate of the highly doped polycrystalline layer is low, so much time is necessary to form the thick polycrystalline layer, and therefore the semiconductor device.